Hematopoietic stem cells (HSCs) are the only cells within the bone marrow that possess the ability to both differentiate to all blood lineages and to self-renew for life. These two properties allow for the proper maintenance of hematopoietic homeostasis, but genetic abnormalities within HSCs can lead to profound negative consequences such as immunodeficiency, anemia, or leukemia. Because the replacement of genetically abnormal HSCs with normal HSCs can correct many of these diseases, it is of critical importance to understand how best to achieve such an exchange while minimizing risk to the patient. One of the most important parameters for the success of bone marrow transplantations is the availability of sufficient numbers of appropriate bone marrow microenvironments for donor HSCs to engraft. The research proposal outlined below describes a series of both classical and technologically novel approaches designed to better understand the functional interactions of HSCs with the highly specific bone marrow microenvironments in which they reside and the potential for therapeutic intervention by capitalizing upon these dynamic interactions. It is anticipated that significant contributions to this field will allow me to transition to an independent faculty position within the next 3 years. Specific Aims: Aim 1: To characterize the kinetics, magnitude, and therapeutic potential of HSC niche emptying and refilling under steady state conditions. Aim 2: To isolate and characterize HSC-supportive niche cells. Aim 3: To localize and visualize HSC homing and engraftment in vivo. Relevance: The hematopoietic stem cell is a remarkable cell type that exists within the bone marrow and is responsible for the production of all red and white blood cells for life. In order for them to function appropriately, they must reside within specific areas, or niches, within the bone marrow. The goal of this proposal is to better characterize the properties of these niches and to determine how best to utilize these niches for bone marrow transplantation.